This invention relates to modulation of inositol trisphosphate (InsP.sub.3) concentration in neurons.
Within the nervous system, information is conveyed from one neuron to another by electrical signals that are generated by the flux of ions, including calcium ions, across the neuronal cell membrane. When certain cell surface receptors are bound, calcium enters the cell through selective channels and may also be released from intracellular stores. The cell surface receptors involved include those that are bound by excitatory amino acids such as glutamate. Glutamate, and other agonists (discussed below), bind metabotropic receptors that are coupled to G proteins, and thereby instigate the biochemical cascade that leads to the release of calcium from intracellular stores.
There are seven immunologically distinct subtypes of metabotropic glutamate receptors (M1-M7). When bound, two of these receptor subtypes, M1 and M5, produce the second messenger InsP.sub.3 by stimulating phosphoinositide-specific phospholipase C (hereinafter, "phospholipase C"), which converts phosphatidylinositol bisphosphate, a lipid located in the plasma membrane, to diacylglycerol and InsP.sub.3 (this reaction is illustrated in FIG. 1).
In addition to L-glutamate, metabotropic receptors are activated by L-aspartate and by the pharmacological agonists quisqualate, ibotenate, and trans-ACPD (trans-(+-1)-1-amino-1,3-cyclopentanedicarboxylate; Schoepp et al., supra). L-aspartate and aspartate analogs also act as agonists for metabotropic receptors expressed by neurons in the brain (Porter et al., Neurosci. Lett., 144:87-89, 1992). In addition to stimulating metabotropic receptors, quisqualate and ibotenate stimulate jonotropic receptors, which are coupled to ion channels (Watkins et al., Trends Pharmacolo. Sci. 11:25-33, 1993). Thus, of the excitatory amino acid receptor agonists, trans-ACPD may be more selective for phosphoinositide-linked metabotropic receptors (Desai and Conn, Neurosci. Lett., 109:157-162, 1990). Pharmacological testing has also shown that L-trans-pyrrolidine-2,4-dicarboxylate and D,L-homocysteate stimulate receptor-coupled phosphoinositide hydrolysis in rat brain tissue (Li and Jope, Biochem. Pharmacol. 38:2781-2787, 1989).
Overstimulation of metabotropic receptors is thought to occur in the course of several neurological disorders. This overstimulation, and the resulting increase in InsP.sub.3 production, increases intracellular calcium to levels that produce severe hyper-functional defects (see, for example, Thomsen et al., J. Neurochem., 62:2492-2495, 1994) and eventual neurotoxicity and death (Berridge, Nature, 361:315-325, 1993; Choi and Rothman, Ann. Rev. Neurosci., 13:171-182, 1990). Specific disorders associated with overstimulation of metabotropic glutamate receptors in the brain include limbic seizures (Tiziano et al., Neurosci. Lett., 162:12-16, 1993) and chronic neurodegenerative disorders such as Alzheimer's disease, Huntington's disease, Parkinson's Disease, and amyotrophic lateral sclerosis (ALS; more commonly known as Lou Gehrig's Disease).
Another type of neuronal cell death, referred to as programmed cell death or apoptosis, may be affected by reduced activity of metabotropic glutamate receptors (Copani et al., J. Neurochem. 64:101-108, 1995). Similarly, inhibition of InsP.sub.3 is thought to mediate neuronal apoptosis by reducing intracellular calcium.